Alarm Going Off After Locking Car - Uk

Hello All. I've had a UK 2.8 CRD diesel since 2010. Recently, about 1 to 3 minutes after locking the car using the key remote the alarm will sound. I can easily switch it off by unlocking the car, again using the key remote. There are no open door warnings on the dash and I've checked the hood/bonnet switch. All seem ok as are all other functions of the car. There are the interior ultrasonic detectors at the front and rear of the car, but I've not found any mention of these in any wiring diagram for the alarm. I also guess that the alarm has a voltage change detection system, but again I can find no information about this.

Has anyone come across this problem before or can anyone suggest where to look?

At the moment I'm locking the side doors manually and the alarm is not armed. But is there a way of manually locking the tailgate?

There was another owner who had a similar problem a while back. Turned out to be an outdoor motion sensor activated light triggering the alarm.

You can manually lock the lift gate. But to do so, you have to remove the small access panel on the inside. It has clips on the bottom and tabs on the top (open from bottom, then slide it down to remove).
Then you will have to reach down to get to the white knob to lock it.
It is tough to get to, but can be done.
Turn the white knob 1/16th turn counter clockwise. This will lock the locking mechanism.
Alternately, you can use the lock button on the door to lock all the doors. Then open the drivers door manually push the lock pin and close the door.

When the alarm goes off. Before you turn it off, take a quick look thru the window and see if there is an "Ajar" message on the cluster.

You can also take a look at the hood switch to see if there is any corrosion on it.
If you have an aftermarket trailer plug, try disconnecting it from the OEM wires. I've seen them have a small internal short which can draw down the battery.
One last suggestion, unplug anything connected to the Cig lighter socket.

Hey GTUK, I found some maybe useful info...
The motion detect feature of the intrusion module can be disabled by depressing the LOCK button on the key fob 3 times within 15 seconds during the arming sequence, (while the security light is still flashing fast). The alarm will make a single short siren chirp as confirmation that the motion detect disable request has been received.

you could try a vehicle computer reset, disconnect the NEGATIVE battery terminal for 10 minutes then reconnect,
or have your vehicle battery checked to make sure it is in good condition, the Nitro does not like a weak battery and can cause all kinds of electrical issues,
visualy check the alarm sensor in the rear headliner is seated correctly (dont try moving it unless it looks like it has been knocked upwards into the headliner) as it comunicates with the front sensor,
also make sure you have nothing hanging in the vehicle which could set off the motion sensor ( air freshners etc ).

Well .... the problem was caused by the alarm siren module which sits behind the front panel somewhere. This contains a back-up battery so that the alarm continues to sound if the main car battery is disconnected. After 10 years of use this back-up battery gave up and caused the alarm to go off immediately it became armed. A new siren module was obtained from Mercedes at GBP 83 and the car is now ok. This might be a problem to come for some of the older Nitro's.

Well .... the problem was caused by the alarm siren module which sits behind the front panel somewhere. This contains a back-up battery so that the alarm continues to sound if the main car battery is disconnected. After 10 years of use this back-up battery gave up and caused the alarm to go off immediately it became armed. A new siren module was obtained from Mercedes at GBP 83 and the car is now ok. This might be a problem to come for some of the older Nitro's.

Click to expand...

The siren module is a Mercedes-Benz Alarm Siren Part No 2198203226. Apparently the batteries are supposed to last 7 years. The battery pack looks like a stack of button cells which I could have replaced myself after opening it up, but it just seemed easier to replace the whole device.

The siren module is a Mercedes-Benz Alarm Siren Part No 2198203226. Apparently the batteries are supposed to last 7 years. The battery pack looks like a stack of button cells which I could have replaced myself after opening it up, but it just seemed easier to replace the whole device.

mine did go off twice last saturday and sunday night, now also was the first time i reversed the car on the driveway, this was like at random times hours after the car was parked, all i could put it down to was maybe our cat jumping off the House roof onto the car roof,

have not parked it that way since and has not gone off again,

so the Nitro has motion detection built in ?
if i went outside now and rocked the car the alarm would go off ?

I don't think it has any vibration detection. My alarm kept going off for no reason and gradually got worse until it started going off as soon as it armed itself after locking the doors with the remote.

As far as I know, what can trigger the alarm is a) the proximity switches on the 4 side doors and the rear door, b) the hood/bonnet pin switch at the front of the car, c) the two interior ultrasonic(?) detectors, d) not sure but it may also have voltage change detection in the alarm.

If you're reasonably sure that it's not being caused by any of the above I would check the backup battery voltage in the siren unit.